1. Field of the Invention
The present invention relates to an evaluation method for polycrystalline silicon. More specifically, the present invention relates to an evaluation method for polycrystalline silicon which may be used as a material for pulling single crystal silicon.
2. Description of Related Art
As a method for producing single crystal silicon, the Czochralski method (hereinafter referred to as the CZ method) is well-known. The CZ method has the advantages that single crystals of a large diameter and high purity can easily be obtained with no transition state or a small number of little lattice defects.
In the CZ method, after washing a polycrystalline silicon piece of extremely high purity, the polycrystalline silicon piece is put into a quartz crucible and melted in a heating furnace. At that time a necessary amount of a conductive impurity (e.g., an additive or a dopant) is added to control, for example, the type of crystal to be prepared. For instance, a P-type crystal is obtained if boron (B) is added whereas an N-type crystal is obtained if phosphorus (P) or antimony (Sb) is added. Also, the resistivity of the crystal may be controlled by changing the amount of the conductive impurity added.
After that, a seed crystal (a single crystal) which is hung by a wire is immersed in the melted silicon and the single crystal is grown by gradually pulling the wire while rotating the single crystal. Single crystal silicon having various diameters and characteristics may be produced by controlling the temperature, the pulling speed and so forth. The crystal grown in this manner becomes a perfect single crystal. The lower the amount of contaminants contained in the polycrystalline silicon used as the material, the less likely that the single crystal produced is subjected to a transition.
However, even if the purity of the polycrystalline silicon is extremely high at first, contaminants, such as metal particles, may attach to the surface of silicon when the polycrystalline silicon is crushed into pieces having a certain particle size. Also, a fine resin particle may attach to the surface of the polycrystalline silicon while being transported. Accordingly, there are cases where fine particles of a metal or a resin are already attached to the surface of polycrystalline silicon pieces when, for instance, a manufacturer of single crystal silicon purchases the polycrystalline silicon from a supplier. For this reason, although the manufacturer washes the polycrystalline silicon beforehand, not all of the contaminants are washed away and some may still remain on the surface.
Since the contaminants attached to the surface of polycrystalline silicon pieces may cause problems, such as crystal defects, in single crystal silicon prepared by the pulling method, it is naturally required to use as clean a polycrystalline silicon piece as possible. However, because the number of particles of contaminants attached to the surface of polycrystalline silicon differ depending on the supplier or product lot, it is necessary to determine the number of particles attached to the polycrystalline silicon before its use, so that it becomes possible to select usable polycrystalline silicon, or use the polycrystalline silicon for a suitable purpose.
Conventionally, the evaluation of the quality of polycrystalline silicon has been carried out by actually preparing single crystal silicon from purchased polycrystalline silicon and measuring, for instance, the density of defects, such as crystal defects, of the single crystal silicon obtained. Accordingly, it takes time to carry out the evaluation procedure and it is difficult to flexibly apply the evaluation results to actual practice, such as the above-mentioned selection of polycrystalline silicon or use for a suitable purpose.
The present invention was achieved in consideration of the above problems and its objectives include providing a method for effectively evaluating the level of contaminants contained in polycrystalline silicon which may be used as a material.